We have ascertained that male mice are able to distinguish between pairs of females that differ only in regard to their major histocompatibility types. This is evident in the phenomena we have called 'Strain Preference' and 'Consistency of Choice'. Strain Preference connotes the MHC-associated propensity of males of a given genotype to mate with females of particular MHC/H-2 types. Consistency of Choice refers to the additional tendencies of particular males to mate, on sequential testing, with females of a particular MHC/C-2 type. We wish to investigate: (i) Whether this system of self-identification and recognition in mice is based on chemo-reception, and if so to determine the nature of the H-2-associated substances responsible; and (ii) Whether the HLA major histocompatibility types of man have similar properties, and can similarly identify individuals. (1) The main purpose of the mouse experiments is to determine whether it is possible to transfer identification by grafting tissues or by inoculation of any biological material. Thus we expect to obtain definitive evidence for or against a chemo-receptive mechanism, and to establish an assay for the putative identifying substances or pheromones. (2) The experiments we have planned to determine whether HLA human types are the source of olfactory identification are based on the use of tracker dogs. The dogs will be trained and tested for their capacity to discriminate between MHC/H-2 types in mice, as a prelude to related tests in man.